From perldata (emphasis mine):

In fact, an identifier within such curlies is forced to be a string, as is any simple identifier within a hash subscript. Neither need quoting. Our earlier example, $days{'Feb'} can be written as $days{Feb} and the quotes will be assumed automatically. But anything more complicated in the subscript will be interpreted as an expression. This means for example that $version{2.0}++ is equivalent to $version{2}++ , not to $version{'2.0'}++